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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #112491
Title: STABLE KNICKPOINTS FORMED IN COHESIVE SEDIMENT

Author
item Bennett, Sean
item Robinson, Kerry
item Simon, Andrew
item Hanson, Gregory

Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: In rivers and streams, vertical step-changes in bed surface elevation or knickpoints have been historically interpreted as unstable landforms. As such, the U.S. Corps of Engineers commonly would construct a grade control structure upstream of a knickpoint to control stream erosion. Geomorphic studies in Mississippi have shown that knickpoints commonly occur along the Yalobusha River system where cohesive clay units are exposed at the bed surface. Detailed examination of these units shows that rates of erosion for these materials can vary widely, suggesting that some knickpoints may be non-migrating and act as natural grade-control structures. An experimental program was designed to construct a non-migrating headcut in an experimental flume and to examine the effects of backwater height on overfall erodibility. A soil was incrementally packed into a flume, and a knickpoint was subjected to a range of flows. Although some bed erosion was observed, the knickpoint remained fixed in position. The highest rates of erosion were observed in the area where the waterfall impacted the bed and when the height of the water downstream of the knickpoint was low. It is postulated herein that along the Yalobusha River knickpoints become destabilized at relatively low flow stages. Thus if a knickpoint can be protected at low flow stages, these stream features may act as natural grade control structures and save the U.S. Corps of Engineers millions of dollars in construction costs.

Technical Abstract: In rivers and streams, vertical step-changes in bed surface elevation or knickpoints have been historically interpreted as unstable landforms. Geomorphic studies in Mississippi have shown that knickpoints commonly occur along the Yalobusha River system where cohesive clay units are exposed at the bed surface. Detailed examination of these units shows that erodibility coefficients can vary widely, suggesting that some knickpoints may be non-migrating and act as natural grade-control structures. An experimental program was designed to construct a non-migrating headcut in an experimental flume and to examine the effects of backwater height on overfall erodibility. A clay loam was incrementally packed into a flume, and a knickpoint was subjected to a range of flows. Although some bed erosion was observed, the knickpoint remained fixed in position. Highest rates of erosion were observed in the area of overfall impingement when flow discharge was high and backwater level was low. Predictive relations for stress at an overfall and an excess shear stress model for rates of erosion were comparable to those observed. It is postulated herein that along the Yalobusha River knickpoints become destabilized at low flow stages when the overfall impinges the bed, thus causing scour, tension cracks, and cantilever mass failure.